10 . 
rUKSIDENTS ADDRESS. 
destroyed ; probably because the natural oxygen of the water M’as 
totally excluded, and its place supplied by poisonous gases. 
The number of the excrements of Entomostraca in the recent 
mud of the Thames is most surprising. In one specimen of mud 
from Hammersmith, Mr. Sorby found more than twenty thousand 
per grain, and the average number at Erith, in August, 1882, was 
above seven thousand, which is equivalent to about two hundred 
thousand per gallon of water at half ebb, from the surface to the 
bottom. This enormous number must represent a large amount of 
sewage material consumed as food. Mr. Sorby goes on to say that 
his observations lead him to the conclusion, that Avhen the amount 
of sewage discharged into a river is not too great, it furnishes food 
for a vast number of animals which perform an important part in 
removing it. On the contrary, if the discharge be too great, it 
may be injurious to them, and thus the process of j)urification 
may cease. Possibly this explains why in certain cases a river, 
which is usually unobjectionable, may occasionally become offensive. 
The same authority then speculates as to whether these larger 
microscopic forms are capable of removing and destroying the still 
smaller germs, such as we are in the habit of associating with 
zymotic disease, and thinks it probable that such is not the case 
except incidentally ; but suggests the probability that the more 
minute Infusoria do consume such germs as a portion of their 
food ; and if so, we should be able to understand how living 
bodies, which could resist any chemical action likely to occur in 
a river, might be destroyed by the digestive process of minute 
animals or plants. 
The number of suspended Diatoms, Desmids, and Alga?, per gallon 
of water is fairly astonishing. In the mud deposited from pure 
rivers they are comparatively few ; but in districts Avhere sewage is 
discharged, the number is vastly increased. Eor instance, iMr. Sorby 
found in the Thames as many as five millions per gallon of water ; 
and concludes that their effect in oxygenating the water must bo very 
important, since, when exposed to light, they would decompose 
carbonic acid, and give off oxygen under circumstances most favour- 
